In recent years, brand-new ultra super critical boilers, which use increased steam temperature and pressure to enhance the boiler efficiency, have been installed worldwide. Specifically, it is also planned that the steam temperature, which has been about 600° C. so far, is to be raised to 650° C. or higher, or further to 700° C. or higher. This is based on the fact that energy saving, effective use of resources, and reduction in CO2 gas emissions for environmental preservation have been issues to be solved and an important industrial policy. This is also because, to a power generation boiler, a reactor for chemical industry, and the like in which a fossil fuel is burnt, an ultra super critical boiler and a reactor that offer high efficiency are advantageous.
The high-temperature and pressure of steam raises the actual operation temperature of high-temperature equipment consisting of thick plates and forgings, which are used as boiler superheater tubes and tubes of reactor for chemical industry, and heat resistant pressurized parts, to 700° C. or higher. Therefore, the material that is used in such a harsh environment for a long period of time is required to have not only excellent high-temperature strength and high-temperature corrosion resistance but also excellent long-term stability of metal micro-structure and creep characteristics.
Accordingly, Patent Documents 1 to 3 disclose heat resistant alloys that contain an increased amount of Cr and Ni and further contain one or more kinds of Mo and W to improve the creep rupture strength, which is one kind of the high-temperature strength.
Further, to meet the increasingly stringent requirement for high-temperature strength characteristics, especially the requirement for creep rupture strength, Patent Documents 4 to 7 disclose heat resistant alloys that contain, by mass percent, 28 to 38% of Cr and 35 to 60% of Ni and utilize the precipitation of an α-Cr phase of body-centered cubic structure consisting mainly of Cr to further improve the creep rupture strength.
On the other hand, Patent Documents 8 to 11 disclose Ni-based alloys that contain Mo and/or W to achieve solid-solution strengthening, and also contain Al and Ti and utilize the precipitation strengthening of a γ′ phase, which is an intermetallic compound, specifically Ni3(Al, Ti) to be allowed being used in the above-described harsh high-temperature environment.
Also, Patent Document 12 discloses a high-Ni austenitic heat resistant alloy in which the addition range of Al and Ti is controlled, and a γ′ phase is precipitated to improve the creep strength.
Further, Patent Documents 13 to 16 disclose Ni-based alloys that contain Co in addition to Cr and Mo to further increase the strength.